The present invention relates generally to fastener-driving tools used to drive fasteners into workpieces, and specifically to combustion-powered fastener-driving tools, also referred to as combustion tools.
Combustion-powered tools are known in the art, and exemplary tools produced by Illinois Tool Works of Glenview, Ill., also known as IMPULSE® brand tools for use in driving fasteners into workpieces, are described in commonly assigned patents to Nikolich U.S. Pat. Re. No. 32,452, and U.S. Pat. Nos. 4,522,162; 4,483,473; 4,483,474; 4,403,722; 5,197,646; 5,263,439; 5,897,043 and 6,145,724 all of which are incorporated by reference herein.
Such tools incorporate a generally pistol-shaped tool housing enclosing a small internal combustion engine. The engine is powered by a canister of pressurized fuel gas, also called a fuel cell. A battery-powered electronic power distribution unit produces a spark for ignition, and a fan located in a combustion chamber provides for both an efficient combustion within the chamber, while facilitating processes ancillary to the combustion operation of the device. Such ancillary processes include: inserting the fuel into the combustion chamber; mixing the fuel and air within the chamber; and removing, or scavenging combustion by-products. The engine includes a reciprocating piston with an elongated, rigid driver blade disposed within a single cylinder body.
A valve sleeve is axially reciprocable about the cylinder and, through a linkage, moves to close the combustion chamber when a work contact element at the end of the linkage is pressed against a workpiece. This pressing action also triggers a fuel-metering valve to introduce a specified volume of fuel into the closed combustion chamber.
Upon the pulling of a trigger switch, which causes the spark to ignite a charge of gas in the combustion chamber of the engine, the combined piston and driver blade is forced downward to impact a positioned fastener and drive it into the workpiece. The piston then returns to its original, or pre-firing position, through differential gas pressures within the cylinder. Fasteners are fed magazine-style into the nosepiece, where they are held in a properly positioned orientation for receiving the impact of the driver blade.
Combustion-powered tools now offered on the market are sequentially operated tools. The tool must be pressed against the work, collapsing the work or workpiece contact element (WCE) before the trigger is pulled for the tool to fire a nail. This contrasts with tools which can be fired in what is known as repetitive cycle operation. In other words, the latter tools will fire repeatedly by pressing the tool against the workpiece if the trigger is held in the depressed mode. These differences manifest themselves in the number of fasteners that can be fired per second for each style tool. The repetitive cycle mode is substantially faster than the sequential fire mode; 4 to 7 fasteners can be fired per second in repetitive cycle as compared to only 2 to 3 fasteners per second in sequential mode.
Effective and complete piston return to the pre-firing position after combustion is required for dependable operation in sequential firing combustion tools as well as repetitive cycle combustion tools. An important factor that limits combustion-powered tools to sequential operation is the manner in which the drive piston is returned to the initial position after the tool is fired. Combustion-powered tools utilize self-generative vacuum to perform the piston return function. Piston return of the vacuum-type requires significantly more time than that of tools that use positive air pressure from the supply line for piston return.
With combustion-powered tools of the type disclosed in the patents listed above, by firing rate and control of the valve sleeve the operator controls the time interval provided for the vacuum-type piston return. The formation of the vacuum occurs following the combustion of the mixture and the exhausting of the high-pressure burnt gases. With residual high temperature gases in the tool, the surrounding lower temperature aluminum components cool and collapse the gases, thereby creating a vacuum. In many cases, the tool operating cycle rate is slow enough, such as in trim applications that vacuum return works consistently and reliably.
However, for those cases where a tool is operated at a much higher cycle rate, the operator can open the combustion chamber early by removing the tool from the workpiece, allowing the valve sleeve to return to a rest position, causing the vacuum to be lost. Without vacuum to move it, piston travel stops before reaching the top of the cylinder. This leaves the driver blade in the guide channel of the nose, thereby preventing the nail strip from advancing. The net result is no nail in the firing channel and no nail fired in the next shot.
Conventional combustion tools using the sequential-fire mode assure adequate closed combustion chamber dwell time with a chamber lockout mechanism that is linked to the trigger. This mechanism holds the combustion chamber closed until the operator releases the trigger, thus taking into account the operator's relatively slow musculature response time. In other words, the physical release of the trigger consumes enough time of the firing cycle to assure piston return. It is disadvantageous to maintain the chamber closed longer than the minimum time to return the piston, as cooling and purging of the tool is prevented.
Piston return in vacuum return combustion tools is the longest single process in the tool's engine cycle, which is defined as the time from when ignition occurs and the piston is returned to the pre-firing position. Times for piston return can range to 75 or even over 100 milliseconds. These times are controlled by the rate and magnitude of vacuum formation. When the tool is operated in a repetitive cycle mode, a faster cycle time is desired and thus less time is available for achieving proper piston return. A piston that does not fully return will prevent the tool from firing properly in a subsequent cycle.
Thus, there is a need for a combustion-powered fastener-driving tool provided with an enhanced piston return which is capable of operating in a repetitive cycle mode, and also which is capable of enhancing operation of sequentially firing combustion-powered tools.